Retrievable formation resistivity tool

ABSTRACT

Systems and methods for downhole communication and measurement utilizing an improved metallic tubular having an elongated body with tubular walls and a central bore adapted to receive a run-in tool. The tubular including slotted stations to provide through-tubular signal transmission and/or reception. Hydraulic isolation between the interior and exterior of the tubular is provided by pressure barrier means at the slotted stations. Sensors and/or sources are mounted on the run-in tool, which is adapted for transmission through a drill string to engage within the tubular in alignment with the slotted stations. A run-in tool configuration includes a modulator for real-time wireless communication with the surface and/or remote downhole tools. A tubular and run-in tool configuration also includes inductive couplers for wireless signal data transfer. A method for measuring a formation characteristic utilizing a run-in tool adapted with an interchangeable end segment for multi-mode downhole transport. Methods for sealing an opening on the surface of a tubular having an elongated body with tubular walls and a central bore.

CROSS-REFERENCES

The present application is a divisional of U.S. patent application Ser.No. 10/355,732, filed Jan. 30, 2003, which is a divisional of U.S.patent application Ser. No. 09/576,271, filed May 22, 2000 now U.S. Pat.No. 6,577,244.

1. BACKGROUND OF THE INVENTION

1.1. Field of the Invention

This invention relates generally to investigation of subsurface earthformations, systems and methods for transmitting and/or receiving asignal through a metallic tubular, and, more particularly, to a devicefor receiving a run-in tool.

1.2. Description of Related Art

Resistivity and gamma-ray logging are the two formation evaluationmeasurements run most often in well logging. Such measurements are usedto locate and evaluate the properties of potential hydrocarbon bearingzones in subsurface formations. In many wells, they are the only twomeasurements performed, particularly in low cost wells and in surfaceand intermediate sections of more expensive wells.

These logging techniques are realized in different ways. A well tool,comprising a number of transmitting and detecting devices for measuringvarious parameters, can be lowered into a borehole on the end of a cableor wireline. The cable, which is attached to some sort of mobileprocessing center at the surface, is the means by which parameter datais sent up to the surface. With this type of wireline logging, itbecomes possible to measure borehole and formation parameters as afunction of depth, i.e., while the tool is being pulled uphole.

Some wells may not be logged because wireline logging is too expensive,when rig time is included in the total cost. Conditioning the well forwireline logging, rigging up the wireline tools, and the time to run thewireline tools in and out require rig time. Horizontal or deviated wellsalso present increased cost and difficulty for the use of wirelinetools.

An alternative to wireline logging techniques is the collection of dataon downhole conditions during the drilling process. By collecting andprocessing such information during the drilling process, the driller canmodify or correct key steps of the operation to optimize performance.Schemes for collecting data of downhole conditions and movement of thedrilling assembly during the drilling operation are known as MeasurementWhile Drilling (MWD) techniques. Similar techniques focusing more onmeasurement of formation parameters than on movement of the drillingassembly are know as Logging While Drilling (LWD). As with wirelinelogging, the use of LWD and MWD tools may not be justified due to thecost of the equipment and the associated service since the tools are inthe hole for the entire time it takes to drill the section.

Logging While Tripping (LWT) presents a cost-effective alternative toLWD and MWD techniques. In LWT, a small diameter “run-in” tool is sentdownhole through the drill pipe, at the end of a bit run, just beforethe drill pipe is pulled. The run-in tool is used to measure thedownhole physical quantities as the drill string is extracted or trippedout of the hole. Measured data is recorded into tool memory versus timeduring the trip out. At the surface, a second set of equipment recordsbit depth versus time for the trip out, and this allows the measurementsto be placed on depth.

U.S. Pat. No. 5,589,825 describes a LWT technique incorporating alogging tool adapted for movement through a drillstring and into adrilling sub. The '825 patent describes a sub incorporating a windowmechanism to permit signal communication between a housed logging tooland the wellbore. The window mechanism is operable between an open andclosed position. A disadvantage of the proposed apparatus is that theopen-window mechanism directly exposes the logging tool to the rugoseand abrasive borehole environment, where formation cuttings are likelyto damage the logging tool and jam the window mechanism. Downholeconditions progressively become more hostile at greater depths. Atdepths of 5,000 to 8,000 meters, bottom hole temperatures of 260° C. andpressures of 170 Mpa are often encountered. This exacerbates degradationof external or exposed logging tool components. Thus, an open-windowstructure is impractical for use in a downhole environment.

UK Patent Application GB 2337546A describes a composite structureincorporated within a drill collar to permit the passage ofelectromagnetic energy for use in measurements during the drillingoperation. The '546 application describes a drill collar having voids orrecesses with embedded composite covers. A disadvantage of the apparatusproposed by the '546 application is the use of composite materials as anintegral part of the drill collar. Fatigue loading (i.e., the bendingand rotating of the drill pipe) becomes an issue in drilling operations.When the drill pipe is subjected to bending or torsion, the shapes ofthe voids or recesses change, resulting in stress failure and poorsealing. The differences in material properties between the metal andcomposite covers are difficult to manage properly where the compositeand metal are required to act mechanically as one piece, such asdescribed in the '546 application. Thus, the increased propensity forfailure under the extreme stresses and loading encountered duringdrilling operations makes implementation of the described structureimpractical.

U.S. Pat. Nos. 5,988,300 and 5,944,124 describe a composite tubestructure adapted for use in a drillstring. The '300 and '124 patentsdescribe a piecewise structure including a composite tube assembled withend-fittings and an outer wrapping connecting the tube with theend-fittings. In addition to high manufacturing costs, anotherdisadvantage of this structure is that the multi-part assembly is moreprone to failure under the extreme stresses encountered during drillingoperations.

U.S. Pat. No. 5,939,885 describes a well logging apparatus including amounting member equipped with coil antennas and housed within a slotteddrill collar. However, the apparatus is not designed for LWT operations.U.S. Pat. Nos. 4,041,780 and 4,047,430 describe a logging instrumentthat is pumped down into a drill pipe for obtaining logging samples.However, the system proposed by the '780 and '430 patents requires thewithdrawal of the entire drill string (for removal of the drill bit)before any logging may be commenced. Thus, implementation of thedescribed system is impractical and not cost effective for manyoperations.

U.S. Pat. No. 5,560,437 describes a telemetry method and apparatus forobtaining measurements of downhole parameters. The '437 patent describesa logging probe that is ejected into the drill string. The logging probeincludes a sensor at one end that is positioned through an aperture in aspecial drill bit at the end of the drill string. As such, the sensorhas direct access to the drill bore. A disadvantage of the apparatusproposed by the '437 patent is the sensor's direct exposure to thedamaging conditions encountered downhole. The use of a small probeprotruding through a small aperture is also impractical for resistivitylogging.

U.S. Pat. No. 4,914,637 describes a downhole tool adapted for deploymentfrom the surface through the drill string to a desired location in theconduit. A modulator on the tool transmits gathered signal data to thesurface. U.S. Pat. No. 5,050,675 (assigned to the present assignee)describes a perforating apparatus incorporating an inductive couplerconfiguration for signal communication between the surface and thedownhole tool. U.S. Pat. No. 5,455,573 describes an inductive couplingdevice for coaxially arranged downhole tools. Downhole techniques havealso been proposed utilizing slotted tubes. U.S. Pat. No. 5,372,208describes the use of slotted tube sections as part of a drill string tosample ground water during drilling. However, none of these proposedsystems relate to through-tubing measurement or signal transfer.

It is desirable to obtain a simplified and reliable LWT system andmethods for locating and evaluating the properties of potentialhydrocarbon bearing zones in subsurface formations. Thus, there remainsa need for an improved LWT system and methods for transmitting and/orreceiving a signal through an earth formation. There also remains a needfor a technique to measure the characteristics of a subsurface formationwith the use of a versatile apparatus capable of providing LWT, LWD orwireline measurements. Yet another remaining need is that of effectivetechniques for sealing apertures on the surface of tubular members usedfor downhole operations.

2. SUMMARY OF THE INVENTION

Systems and methods are provided utilizing an improved downhole tubularhaving an elongated body with tubular walls and a central bore adaptedto receive a run-in tool. The tubular has at least one slot formed inits wall to provide for continuous passage of a signal (e.g.,electromagnetic energy) that is generated or received respectively by asource or sensor mounted on the run-in tool. The tubular also includes apressure barrier within the central bore to maintain hydraulic integritybetween the interior and exterior of the tubular at the slotted station.The tubular and run-in tool combinations provide systems and methods fordownhole signal communication and formation measurement through ametallic tubular. A technique for measuring a formation characteristicutilizing a run-in tool adapted with a multi-mode end segment isprovided. Techniques are also provided for effectively sealing openingson the surface of tubular members.

In one aspect of the invention, run-in tools equipped with electronics,sensors, sources, memory, power supply, CPU, batteries, ports,centralizers, and a clock, are provided for deployment through andengagement within a downhole tubular.

In another aspect of the invention, antenna configurations of the run-intool are provided.

In another aspect of the invention, slotted-tubular/run-in toolconfigurations are provided for downhole signal communication andmeasurement.

In another aspect of the invention, pressure barrier configurations areprovided for maintaining the hydraulic integrity of the tubulars at theslotted stations.

In another aspect of the invention, slot-insert configurations areprovided for the slotted tubular.

In another aspect of the invention, antenna-shielding configurations areprovided for focusing the electromagnetic energy generated by theantennas of the run-in tool.

In another aspect of the invention, a run-in tool including a modulatorfor real-time signal/data communication is provided.

In another aspect of the invention, a run-in tool configuration forwireless communication with a remote downhole tool is provided.

In another aspect of the invention, a run-in tool and tubularconfiguration for determining formation porosity utilizing nuclearmagnetic resonance techniques is provided.

In another aspect of the invention, run-in tool and tubularconfigurations for determining formation density utilizing gamma-raytechniques are provided.

In another aspect of the invention, run-in tool and tubularconfigurations for determining formation resistivity utilizingelectromagnetic propagation techniques are provided.

In another aspect of the invention, run-in tool and tubularconfigurations including inductive couplers are provided for downholesignal communication and measurement.

3. BRIEF DESCRIPTION OF THE DRAWINGS

Other aspects and advantages of the invention will become apparent uponreading the following detailed description and upon reference to thedrawings in which:

FIG. 1 is a schematic diagram of a run-in tool in accord with theinvention.

FIG. 2 a is a cross-sectional view of a run-in tool showing an antennawith associated wiring and passages in accord with the invention.

FIG. 2 b is a schematic diagram of a shield structure surrounding anantenna on the run-in tool in accord with the invention.

FIG. 3 is a schematic diagram of a tubular member with slotted stationsin accord with the invention.

FIGS. 4 a and 4 b are schematic diagrams of a run-in tool engaged withina tubular member in accord with the invention.

FIG. 5 graphically illustrates the relationship between the slotdimensions of a tubular segment of the invention and the attenuation ofpassing electromagnetic energy.

FIG. 6 is a schematic diagram of a run-in tool with a centralizerconfiguration in accord with the invention.

FIG. 7 a is a cross-sectional view of a tubular member with a pressurebarrier configuration in accord with the invention.

FIG. 7 b is a cross-sectional view of a three-slotted tubular member ofFIG. 7 a along line A-A.

FIG. 8 a is a cross-sectional view of a tubular member with anotherpressure barrier configuration in accord with the invention.

FIG. 8 b is a cross-sectional view of a three-slotted tubular member ofFIG. 8 a along line B-B.

FIG. 9 a is a cross-sectional view of a run-in tool positioned inalignment with a pressure barrier configuration in accord with theinvention.

FIG. 9 b is a top view of the run-in tool and pressure barrierconfiguration of FIG. 9 a.

FIG. 10 is a cross-sectional view of a pressure barrier and tubularmember configuration in accord with the invention.

FIG. 11 is a cross-sectional view of a slotted tubular member with aninsert, seal, and retaining sleeve in accord with the invention.

FIGS. 12 a and 12 b 1-3 are cross-sectional views and cut-awayperspectives of a slotted tubular station with a tapered slot and acorresponding tapered insert in accord with the invention.

FIG. 13 a is a schematic diagram of a run-in tool and antenna eccenteredwithin a tubular member in accord with the invention.

FIGS. 13 b and 13 c are schematic diagrams of a run-in tool and antennasurrounded by a focusing shield and respectively showing the shield'seffect on the magnetic and electric fields in accord with the invention.

FIG. 14 is a top view of a shielding structure formed within the bore ofthe tubular member in accord with the invention.

FIG. 15 is a schematic diagram of a shielding structure formed by acavity within the run-in tool in accord with the invention.

FIG. 16 is a schematic diagram of a run-in tool including a modulatorengaged within a tubular member in accord with the invention.

FIG. 17 is a schematic diagram of the run-in tool configuration of FIG.16 as used for real-time wireless communication with a remote downholetool in accord with invention.

FIG. 18 is a schematic diagram of a run-in tool configuration forporosity measurements utilizing magnetic nuclear resonance techniques inaccord with the invention.

FIGS. 19 a and 19 b are schematic diagrams of run-in tool antennaconfigurations within tubular members in accord with the invention.

FIG. 20 shows schematic diagrams of a tubular member and run-in toolconfiguration with inductive couplers in accord with the invention.

FIGS. 21 a and b show a top view and a schematic diagram and of aneccentered run-in tool and tubular member with inductive couplers inaccord with the invention.

FIGS. 22 a and 22 b are schematic diagrams of an inductive couplerconfiguration within a run-in tool and tubular member in accord with theinvention.

FIG. 23 is a cross-sectional view of an inductive coupler and shieldconfiguration mounted within a tubular member in accord with theinvention.

FIG. 24 is a schematic diagram of a simplified inductive coupler circuitin accord with the invention.

FIG. 25 is a flow chart illustrating a method for transmitting and/orreceiving a signal through an earth formation in accord with theinvention.

FIG. 26 is a flow chart illustrating a method for measuring acharacteristic of an earth formation surrounding a borehole in accordwith the invention.

FIG. 27 is flow chart illustrating a method for sealing an opening onthe surface of a tubular member in accord with the invention.

FIG. 28 is a flow chart illustrating a method for sealing a fullypenetrating opening on surface of a tubular member in accord with theinvention.

4. DETAILED DESCRIPTION ON SPECIFIC EMBODIMENTS

In the interest of clarity, not all features of actual implementationare described in this specification. It will be appreciated thatalthough the development of any such actual implementation might becomplex and time-consuming, it would nevertheless be a routineundertaking for those of ordinary skill in the art having the benefit ofthis disclosure.

The apparatus of the invention consists of two main assets, a run-intool (RIT) and a drill collar. Henceforth, the drill collar will bereferred to as the sub.

4.1 RIT

FIG. 1 shows an embodiment of the RIT 10 of the invention. The RIT 10 isan elongated, small-diameter, metal mandrel that may contain one or moreantennas 12, sources, sensors [sensor/detector are interchangeable termsas used herein], magnets, a gamma-ray detector/generator assembly,neutron-generating/detecting assembly, various electronics, batteries, adownhole processor, a clock, a read-out port and recording memory (notshown).

The RIT 10 does not have the mechanical requirements of a drill collar.Thus, its mechanical constraints are greatly reduced. The RIT 10 has alanding mechanism (stinger) 14 on the bottom end and a fishing head 16on the top. The fishing head 16 allows for the RIT 10 to be captured andretrieved from within a sub with the use of a conventional extractiontool such as the one described in U.S. Pat. No. 5,278,550 (assigned tothe present assignee). An advantage of the fishable RIT 10 assembly is areduction of Lost-In-Hole costs.

As shown in FIG. 2 a, each antenna 12 on the RIT 10 consists ofmulti-turn wire loops encased in fiberglass-epoxy 18 mounted in a groovein the RIT 10 pressure housing and sealed with rubber over-molding 20. Afeed-through 22 provides a passage for the antenna 12 wiring, leading toan inner bore 24 within the RIT 10. Each antenna 12 may be activated toreceive or transmit an electromagnetic (EM) signal as known in the art.

The antennas 12 radiate an azimuthal electric field. Each antenna 12 ispreferably surrounded by a stainless-steel shield 26 (similar to thosedescribed in U.S. Pat. No. 4,949,045, assigned to the present assignedto the present assignee) that has one or more axial slots 28 arrayedaround the shield 26 circumference. FIG. 2 b shows the axial slots 28distributed around the circumference of the shield 26. The shields 26are short-circuited at the axial ends into the metal mandrel body of theRIT 10. These shields 26 permit transverse electric (TE) radiation topropagate through while blocking transverse magnetic (TM) and transverseelectromagnetic (TEM) radiation. The shields 26 also protect theantennas 12 from external damage. The RIT 10 electronics and sensorarchitecture resembles that described in U.S. Pat. No. 4,899,112(assigned to the present assignee).

4.2 Sub

FIG. 3 shows an embodiment of a sub 30 of the invention. The sub 30 hasan elongated body with tubular walls and a central bore 32. The sub 30contains neither electronics nor sensors and is fully metallic,preferably formed from the stainless steel. It is part of the normalbottom hole assembly (BHA), and it is in the hole with the drill stringfor the duration of the bit run. The sub 30 has normal threaded oilfieldconnections (pin and box) at each end (not shown).

The sub 30 includes one or more stations 36 with one or more axial slots38 placed along the tubular wall. Each elongated axial slot 38 fullypenetrates the tubular wall of the sub 30 and is preferably formed withfully rounded ends. Stress modeling has shown that rather long slots 38may be formed in the sub 30 walls while still maintaining the structuralintegrity of the sub 30. Stress relief grooves 40 may be added to the ODof the sub 30, in regions away from the slot(s) 38, to minimize thebending moment on the slot(s) 38.

Each slot 38 provides a continuous channel for electromagnetic energy topass through the sub 30. The slots 38 block TM radiation but allow thepassage of TE radiation, albeit with some attenuation. The degree ofattenuation of TE fields by the sub 30 depends on factors such asfrequency, the number of slots, slot width, slot length, collar OD andID, and the location and dimensions of the RIT 10 antenna. For example,FIG. 5 shows the sub 10 attenuation measured at 400 kHz with a 25-turn1.75-inch diameter coil centered in 3.55-inch ID, 6.75-inch OD subs 30with one or two slots 38 of different lengths and widths. As evidentfrom FIG. 5, adding more slots 38 and making the slots longer or widerdecreases the attenuation. However, with only one or two 0.5-inch wide6-8 inch long slots 38, the sub 30 attenuation is already ˜15 dB, whichis sufficiently low for many applications.

In operation, the RIT 10 is pumped down and/or lowered through thedrillstring on cable at the end of the bit run and engaged inside thesub 30. The RIT 10 is received by a landing “shoe” 42 within the centralbore 32 of the sub 30, as shown in FIG. 4 a. FIG. 4 b, shows how the RIT10 is located in the sub 30 so that each antenna 12, source, or sensoris aligned with a slot 38 in the sub 30. The landing shoe 42 preferablyalso has a latching action to prevent any axial motion of the RIT 10once it is engaged inside the sub 30.

Turning to FIG. 6, an embodiment of the invention includes a centralizer44, which serves to keep the RIT 10 centered and stable within the sub30, lowering shock levels and reducing the effects of tool motion on themeasurement. One or more centralizers 44 may be mounted within thecentral bore 32 to constrain the RIT 10 and keep it from hitting the IDof the sub 30. One or more spring-blades 46 may also be mounted toextend from the centralizer 44 to provide positioning stability for theRIT 10. The spring-blades 46 are compressed against the RIT 10 when itis engaged within the sub 30. Bolts 48 with O-ring seals 50 may be usedto hold the centralizer(s) 44 in the sub 30 while preserving thepressure barrier between the ID and the OD of the sub 30.

Alternatively, the centralizer 44 may be mounted on the RIT 10 ratherthan on the sub 30 (See FIG. 16). In this case, the centralizer 44 maybe configured to remain in a retracted mode during the trip down, and toopen when the RIT 10 lands in the sub 30. It will be understood thatother centralizer 44 configurations may be implemented with theinvention as known in the art.

The RIT 10 and sub 30 have EM properties similar to a coaxial cable,with the RIT 10 acting as the inner conductor, and the sub 30 acting asthe outer conductor of a coaxial cable. If the drilling mud isconductive, then the “coax” is lossy. If the drilling mud is oil based,the “coax” will have little attenuation. Parasitic antenna 12 couplingmay take place inside of the sub 30 between receiver-receiver ortransmitter-receiver. As described above, the shields 26 surrounding theantennas 12 are grounded to the mandrel of the RIT 10 to minimizecapacitive and TEM coupling between them. Electrically balancing theantennas 12 also provides for TEM coupling rejection. The centralizers44 may also be used as a means of contact to provide radio-frequency(rf) short-circuits between the RIT 10 and the sub 30 to preventparasitic coupling. For example, small wheels with sharp teeth may bemounted on the centralizers 44 to ensure a hard short between the RIT 10and the sub 30 (not shown).

4.3 Pressure Barrier

Since each slot 38 fully penetrates the wall of the sub 30, aninsulating pressure barrier is used to maintain the differentialpressure between the inside and the outside of the sub 30 and tomaintain hydraulic integrity. There are a variety of methods forestablishing a pressure barrier between the sub 30 ID and OD at theslotted station 36.

Turning to FIG. 7 a, an embodiment of a sub 30 with a pressure barrierof the invention is shown. A cylindrical sleeve 52 is positioned withinthe central bore 32 of the sub 30 in alignment with the slot(s) 38. Thesleeve 52 is formed of a material that provides transparency to EMenergy. Useable materials include the class of polyetherketonesdescribed in U.S. Pat. No. 4,320,224, or other suitable resins. VictrexUSA, Inc. of West Chester, Pa. manufactures one type called PEEK.Another usable compound is known as PEK. Cytec Fiberite, Greene Tweed,and BASF market other suitable thermoplastic resin materials. Anotheruseable material is Tetragonal Phase Zirconia ceramic (TZP),manufactured by Coors Ceramics, of Golden, Colo. It will be appreciatedby those skilled in the art that these and other materials may becombined to form a useable sleeve 52.

PEK and PEEK can withstand substantial pressure loading and have beenused for harsh downhole conditions. Ceramics can withstand substantiallyhigher loads, but they are not particularly tolerant to shock.Compositions of wound PEEK or PEK and glass, carbon, or KEVLAR may alsobe used to enhance the strength of the sleeve 52.

A retainer 54 and spacer 56 are included within the central bore 32 tosupport the sleeve 52 and provide for displacement and alignment withthe slots 38. The sleeve 52 is positioned between the retainer 54 andspacer 56, which are formed as hollow cylinders to fit coaxially withinthe central bore 32. Both are preferably made of stainless steel. Theretainer 54 is connected to the sleeve 52 at one end, with the sleeve 52fitting coaxially inside the retainer 54. As the differential pressureincreases within the ID of the sub 30 during operation, the sleeve 52takes the loading, isolating the sub 30 from the pressure in the slottedregion. Hydraulic integrity is maintained at the junction between thesleeve 52 and retainer 54 by an O-ring seal 53. A fitted “key” 55 isused to engage the sleeve 52 to the retainer 54, preventing one fromrotating relative to the other (See FIG. 7 a blow-up). An index pin 57is fitted through the sub 30 and engaged to the free end of the retainer54 to prevent the retainer from rotating within the bore 32 of the sub30. O-rings 59 are also placed within grooves on the OD of the retainer54 to provide a hydraulic seal between the retainer 54 and the sub 30.

In operation, the internal sleeve 52 will likely undergo axial thermalexpansion due to high downhole temperatures. Thus, it is preferable forthe sleeve 52 to be capable of axial movement as it undergoes thesechanges in order to prevent buckling. The spacer 56 consists of an innercylinder 60 within an outer cylinder 62. A spring 64 at one end of theOD of the inner cylinder 60 provides an axial force against the outercylinder 62 (analogous to an automotive shock absorber). The outercylinder 62 is connected to the sleeve 52 using the key 55 and O-ringseal 53 at the junction as described above and shown in the blow-up inFIG. 7 a. The spring-loaded spacer 56 accounts for differential thermalexpansion of the components. The sub 30 embodiment of FIG. 7 a is shownconnected to other tubular members by threaded oilfield connections 70.

For purposes of illustration, a sub 30 with only one slot 38 is shown inFIG. 7 a. Other embodiments may include several sleeves 52interconnected in the described manner to provide individual pressurebarriers over multiple slotted stations 36 (not shown). With thisconfiguration, only two O-ring 53 seals to the ID of the sub 30 are usedover the entire slotted array section. This minimizes the risk involvedwith dragging the O-rings 53 over the slots 38 during assembly orrepair. FIG. 7 b shows a cross-section of the sub 30 (along line A-A ofFIG. 7 a) with a three-slot 38 configuration.

FIG. 8 a shows another embodiment of a sub 30 with a pressure barrier ofthe invention. In this embodiment, the spring-loaded spacer 56 maintainsthe outer cylinder 62 abutted against the sleeve 52 and O-rings 68 areplaced within grooves on the OD of the sleeve 52, preferably at bothends of the slot 38. The retainer 54 rests at one end against a shoulderor tab 58 formed on the wall of the central bore 32. FIG. 8 b shows across-section of the sub 30 (along line B-B of FIG. 8 a) with athree-slot 38 configuration.

In another embodiment of a pressure barrier of the invention, a sleeve52 made out of PEEK or PEK, or glass, carbon, or KEVLAR filled versionsof these materials, may be bonded to a metal insert (not shown), wherethe insert contains O-rings to seal against the sub 30 as describedabove. The metal insert could be mounted within the sub 30 as describedabove or with the use of fastener means or locking pins (not shown). Thesleeve material may also be molded or wrapped onto the supportinginsert. The fibers in the wrapped material can also be aligned toprovide additional strength.

FIG. 9 a shows another embodiment of a pressure barrier of theinvention. In this embodiment, the cylindrical sleeve 52 is held inalignment with the slot(s) 38 by a metal retainer 72. The retainer 72may be formed as a single piece with an appropriate slot 74 cut into itfor signal passage as shown, or as independent pieces supporting thesleeve 52 at the top and bottom (not shown). The retainer 72 may beconstrained from axial movement or rotation within the sub 30 by any ofseveral means known in the art, including an index-pin mechanism or akeyed-jam-nut type arrangement (not shown). The slot 38 may also befilled with a protective insert as will be further described below. Inoperation, a RIT 10 is positioned within the sub 30 such that theantenna 12 is aligned with the slot(s) 38.

As shown in FIG. 9 b, the retainer 72 is formed such that it extendsinto and reduces the ID of the sub 30 to constrain the RIT 10. Mudflowoccurs through several channels or openings 76 in the retainer 72 andthrough the annulus 78 between the RIT 10 and the retainer 72. Theretainer 72 in effect acts as a centralizer to stabilize the RIT 10 andto keep it from hitting the ID of the sub 30, lowering shock levels andincreasing reliability.

FIG. 10 shows another embodiment of a pressure barrier of the invention.A sub 30 may be formed with a shop joint 80 so that the sleeve 52 can beinserted within the central bore 32. The sleeve 52 is formed asdescribed above and provides a hydraulic seal using O-rings 82 withingrooves at both ends on the OD of the sleeve 52. The sleeve 52 isrestrained from axial movement within the central bore 32 by a lip 84formed on one end of the two-piece sub 30 and by the end of the matchingsub 30 joint. Since the sleeve 52 sits flush within a recess 86 in theID of the sub 30, this configuration offers unrestricted passage to alarge diameter RIT 10. This configuration also provides easy access tothe sleeve 52 and slot(s) 38 for maintenance and inspection.

Turning to FIG. 11, another embodiment of a pressure barrier of theinvention is shown. The slot 38 in the sub 30 is three-stepped,preferably with fully rounded ends. One of the steps provides a bearingshoulder 90 for an insert 92, and the other two surfaces form thegeometry for an O-ring groove 94 in conjunction with the insert 92. Amodified O-ring seal consists of an O-ring 96 stretched around theinsert 92 at the appropriate step, with metal elements 98 placed onopposite sides of the O-ring 96. The metal elements 98 are preferably inthe form of closed loops.

The sleeve 52 may be fitted within the sub 30 with one or more O-rings(not shown) to improve hydraulic integrity as described above. As shownin FIG. 11, the sleeve 52 may also have a slot 100 penetrating its wallto provide an unobstructed channel for any incoming or outgoing signal.The sleeve 52 may have a matching slot 100 for every slot 38 in the sub30.

the insert 92 and sleeve 52 are preferably made of the dielectricmaterials described above to permit the passage of EM energy. However,if the sleeve 52 is configured with a slot 100, the sleeve 52 may beformed from any suitable material.

If the sleeve 52 is configured with a slot 100, the internal pressure ofthe sub 30 may push the insert 92 outward. The bearing shoulder 52 takesthis load. As the internal pressure increases, the O-ring 96 pushes themetal elements 98 against an extrusion gap, which effectively closes offthe gap. As a result, there is no room for extrusion of the O-ring 96.Since the metal is much harder than the O-ring material, it does notextrude at all. The modified geometry therefore creates a scenario wherea soft element (the O-ring) provides the seal and a hard element (themetal loop) prevents extrusion, which is the ideal seal situation. Inthe event of pressure reversal, the sleeve 52 captures the insert 92 inthe slot 38, preventing the insert 92 from being dislodged.

Other pressure barrier configurations may be implemented with theinvention. One approach is the use of several individual sleeves 52connected together by other retaining structures and restrained by apressure-differential seal or a jam-nut arrangement (not shown). Anotherapproach is the use of a long sleeve 52 to span multiple slottedstations 38 (not shown). Still another approach is the use of a sleeve52 affixed to the OD of the sub 30 over the slotted region, or acombination of an interior and exterior sleeve 52 (not shown).

4.4 Slot Inserts

While the slotted stations of the invention are effective with fullyopen and unblocked slots 38, the operational life of the assembly may beextended by preventing debris and fluids from entering and eroding theslots 38 and the insulating sleeve 52. The slots 38 could be filled withrubber, an epoxy-fiberglass compound, or another suitable fillermaterial to keep fluids and debris out while permitting signal passage.

An embodiment of a sub 30 with a tapered slot 38 is shown in FIG. 12 a.The slot 38 is tapered such that the outer opening W₁ is narrower thanthe inner opening W₂, as shown in FIG. 12 b. A tapered wedge 88 ofinsulating material (e.g., fiberglass epoxy) is inserted within thetapered slot 38. The wedge 88 may be bonded into the sub 30 with rubber.The rubber layer surrounds the wedge 88 and bonds it into the sub 30. Anannulus of rubber may also be molded on the interior and/or exteriorsurface of the sub 30 to seal the wedge 88 within the slot 38.

4.5 Focusing Shield Structures

Measurements of the attenuation of the TE radiation from a simplecoil-wound antenna 12 through a single slot 38 of reasonable dimensionsshow that the TE field is notably attenuated. This attenuation can bereduced, however, by using shielding around the antenna 12 to focus theEM fields into the slot 38.

Turning to FIG. 13 a, an antenna 12 consisting of 25 turns of wire on a1.75-inch diameter bobbin was mounted on a 1-inch diameter metal RIT 10and positioned fully eccentered radially inside the bore of a 3.55-inchID, 6.75-inch OD sub 30 against the slot 38 and centered vertically onthe slot 38. The measured attenuation of the TE field between 25 kHz-2MHz was a nearly constant 16.5 dB.

Turning to FIG. 13 b, the same measurement was performed with theantenna 12 inside a thin shield 102 formed of a metallic tube with a0.5-inch wide, 6-inch long slot 104 aligned with the slot 38 in the sub30 (not shown). The antenna 12 was fully surrounded by the shield 102except for the open slot 104 and placed inside the sub 30.

The attenuation with this assembly in the same sub 30 was 11.8 dB, areduction of the attenuation of nearly 5 dB. FIG. 13 b and 13 crespectively show how the shield 102 affects the magnetic and electricfields. The attenuation due to this shield 102 alone is minimal.

FIG. 14 shows another embodiment of a shielding structure of theinvention. In this embodiment, the central bore 32 of the sub 30 isconfigured with a bracket structure 106 that serves as a focusing shieldby surrounding the antenna 12 when the RIT 10 is engaged within the sub30.

FIG. 15 shows another embodiment of a shielding structure of theinvention. The mandrel of the RIT 10 has a machined pocket or cavity 108in its body. A coil antenna 12 wound on a bobbin 110 made of dielectricmaterial is mounted within the cavity 108. A ferrite rod may replace thedielectric bobbin 110. With this configuration, the body of the RIT 10itself serves as a focusing shield. The hydraulic integrity of the RIT10 is maintained by potting the antenna 12 with fiberglass-epoxy,rubber, or another suitable substance. The attenuation of a coil antenna12 having 200 turns on a 0.875-inch diameter bobbin was measured forthis assembly mounted the same way as described above in the same sub30. The measured attenuation was only ˜7 dB. It will be appreciated bythose skilled in the art that other types of sources/sensors may behoused within the cavity 108 of the RIT 10.

4.6 RIT/Sub Configurations

FIG. 16 shows another embodiment of the invention. A sub 30 of theinvention is connected to another tubular 111 forming a section of adrillstring. The RIT 10 includes an antenna 12, a stinger 14 at thelower end, and a fishing head 16 at the top end. The stinger 14 isreceived by the landing shoe 42 on the sub 30, which serves to align theantenna 12 with the slotted station 36. As above, the RIT 10 of thisembodiment includes various electronics, batteries, a downholeprocessor, a clock, a read-out port, memory, etc. (not shown) in apressure housing. The RIT 10 may also incorporate various types ofsources/sensors as known in the art.

4.6.1 RIT with Modulator

The RIT 10 of FIG. 16 is also equipped with a modulator 116 for signalcommunication with the surface. As known in the art, a useable modulator116 consists of a rotary valve that operates on a continuous pressurewave in the mud column. By changing the phase of the signal (frequencymodulation) and detecting these changes, a signal can be transmittedbetween the surface and the RIT 10. With this configuration, one cansend the RIT 10 through the drillstring to obtain measurement data(e.g., resistivity or gamma-ray counts) of formation characteristics andto communicate such data to the surface in real-time. Alternatively, allor some of the measurement data may be stored downhole in the RIT 10memory for later retrieval. The modulator 116 may also be used to verifythat the RIT 10 is correctly positioned in the sub 30, and thatmeasurements are functioning properly. It will be appreciated by thoseskilled in the art that a modulator 116 assembly may be incorporatedwith all of the RIT/sub implementations of the invention.

FIG. 17 shows another embodiment of the invention. The subs 30 and RITs10 of the invention may be used to communicate data and/or instructionsbetween the surface and a remote tool 112 located along the drillstring. For purposes of illustration, the tool 112 is shown with a bitbox 113 at the bottom portion of a drive shaft 114. The drive shaft 114is connected to a drilling motor 115 via an internal transmissionassembly (not shown) and a bearing section 117. The tool 112 also has anantenna 12 mounted on the bit box 113. The motor 115 rotates the shaft114, which rotates the bit box 113, thus rotating the antenna 12 duringdrilling.

With the configuration of FIG. 17, the RIT 10 may be engaged within thesub 30 at the surface or sent through the drill string when the sub 30is at a desired downhole position. Once engaged, a wirelesscommunication link may be established between the antenna 12 on the RIT10 and the antenna 12 on the tool 112, with the signal passing throughthe slotted station 36. In this manner, real-time wireless communicationbetween the surface and the downhole tool 112 may be established. Itwill be appreciated by those skilled in the art that other types ofsensors and/or signal transmitting/receiving devices may be mounted onvarious types of remote tools 112 for communication with correspondingdevices mounted on the RIT 10.

4.6.2 Nuclear Magnetic Resonance Sensing

It is known that when an assembly of magnetic moments such as those ofhydrogen nuclei are exposed to a static magnetic field they tend toalign along the direction of the magnetic field, resulting in bulkmagnetization. By measuring the amount of time for the hydrogen nucleito realign their spin axes, a rapid nondestructive determination ofporosity, movable fluid, and permeability of earth formations isobtained. See A. Timur, Pulsed Nuclear Magnetic Resonance Studies ofPorosity, Movable Fluid, and Permeability of Sandstones, JOURNAL OFPETROLEUM TECHNOLOGY, June 1969, p. 775. U.S. Pat. No. 4,717,876describes a nuclear magnetic resonance well logging instrument employingthese techniques.

A determination of formation porosity from magnetic resonance may beobtained with a non-magnetic sub 30 of the invention as shown in FIG.18. The sub 30 can be formed of the typical high-strength non-magneticsteel used in the industry. The RIT 10 contains the electronics,batteries, CPU, memory, etc., as described above. Opposing permanentmagnets 118 contained in the RIT 10 provide the magnetic field. A rfcoil 120 is mounted between the magnets 118 for generating a magneticfield in the same region to excite nuclei of the formation vicinity. Thedesign of the rf coil 120 is similar to the antennas 12 described abovein being a multi-turn loop antenna with a central tube for through wiresand mechanical strength. The permanent magnets 118 and rf coil 120 arepreferably housed in a non-magnetic section of the sub 30 that has axialslots 38 with a pressure barrier (not shown) of the invention.

With a non-magnetic sub 30, the static magnetic fields B₀ from thepermanent magnets 118 penetrate into the surrounding formation to excitethe nuclei within the surrounding formation. The coil 120 in the RIT 10provides a rf magnetic field B₁, which is perpendicular to B₀ outside ofthe sub 30. The rf coil 120 is positioned in alignment with the axialslot(s) 38 in the sub 30.

A magnetic resonance measurement while tripping may be more complicatedin comparison to propagation resistivity measurements due to variousfactors, including: an inherently lower signal-to-noise ratio, permanentmagnet form factors, rf coil efficiency, high Q antenna turning, highpower demands, and a slower logging speed.

4.6.3 Gamma-Ray Measurement

It is known that gamma ray transport measurements through a formationcan be used to determine its characteristics such as density. Theinteraction of gamma rays by Compton scattering is dependent only uponthe number density of the scattering electrons. This in turn is directlyproportional to the bulk density of the formation. Conventional loggingtools have been implemented with detectors and a source of gamma rayswhose primary mode of interaction is Compton scattering. See U.S. Pat.No. 5,250,806, assigned to the present assignee. Gamma ray formationmeasurements have also been implemented in LWT technology. See loggingwhile tripping cuts time to run gamma ray, OIL & GAS JOURNAL, June 1996,pp. 65-66. The present invention may be used to obtain gamma-raymeasurements as known in the art, providing advantages over knownimplementations.

The subs 30 of the invention provide the structural integrity requiredfor drilling operations while also providing a low-density channel forthe passage of gamma rays. Turning to FIG. 4 b, this configuration isused to illustrate a gamma-ray implementation of the invention. In thisimplementation, a RIT 10 is equipped with a gamma-ray source andgamma-ray detectors (not shown) of the type known in the art anddescribed in the '806 patent. The antennas 12 of FIG. 4 b would bereplaced with a gamma-ray source and gamma-ray detectors (not shown).

Two gamma-ray detectors are typically used in this type of measurement.The gamma-ray detectors are placed on the RIT 10 at appropriate spacingsfrom the source as known in the art. The slotted stations 36 are alsoappropriately placed to match the source and detector positions of theRIT 10. Calibration of the measurement may be required to account forthe rays transmitted along the inside of the sub 30. The gamma-raydetectors may also be appropriately housed within the RIT 10 to shieldthem from direct radiation from the source as known in the art.

Turning to FIG. 14, this configuration is used to illustrate anothergamma-ray implementation of the invention. With the RIT 10 equipped withthe described gamma-ray assembly and eccentered toward the slots 38,this configuration will capture the scattered gamma rays moreefficiently and provide less transmission loss.

4.6.4 Resistivity Measurement

The invention may be used to measure formation resistivity usingelectromagnetic propagation techniques as known in the art, includingthose described in U.S. Pat. Nos. 5,594,343 and 4,899,112 (both assignedto the present assignee). FIGS. 19 a and 19 b show two RIT 10/sub 30configurations of the invention. A pair of centrally located receiverantennas Rx are used to measure the phase shift and attenuation of EMwaves. Look-up tables may be used to determine phase shift resistivityand attenuation resistivity. Transmitter antennas Tx are placed aboveand below the receiver antennas Rx, either in the configuration shown inFIG. 19 a, which has two symmetrically placed transmitter antennas Tx,or in the configuration shown in FIG. 19 b, which has severaltransmitter antennas Tx above and below the receiver antennas Rx. Thearchitecture of FIG. 19 a can be used to make a borehole compensatedphase-shift and attenuation resistivity measurement, while the multipleTx spacings of FIG. 19 b can measure borehole compensated phase-shiftand attenuation with multiple depths of investigation. It will beappreciated by those skilled in the art that other source/sensorconfigurations and algorithms or models may be used to make formationmeasurements and determine the formation characteristics.

4.7 Inductively-Coupled RIT/Sub

Turning to FIG. 20, other embodiments of a sub 30 and RIT 10 of theinvention are shown. The sub 30 contains one or more integral antennas12 mounted on the OD of the elongated body for transmitting and/orreceiving electromagnetic energy. The antennas 12 are embedded infiberglass epoxy, with a rubber over-molding as described above. The sub30 also has one or more inductive couplers 122 distributed along itstubular wall.

The RIT 10 has a small-diameter pressure housing such as the onedescribed above, which contains electronics, batteries, downholeprocessor, clocks, read-out port, recording memory, etc., and one ormore inductive couplers 122 mounted along its body.

As shown in FIG. 21, the RIT 10 is eccentered inside the sub 30 so thatthe inductive coupler(s) 122 in the RIT 10 and the inductive coupler(s)122 in the sub 30 are in close proximity. The couplers 120 consist ofwindings formed around a ferrite body as known in the art. Feed-throughs124 connect the antenna 12 wires to the inductive coupler 122 located ina small pocket 126 in the sub 30. A metal shield 128 with vertical slotscovers each antenna 12 to protect it from mechanical damage and providethe desired electromagnetic filtering properties as previouslydescribed. Correctly positioning the RIT 10 inside the sub 30 improvesthe efficiency of the inductive coupling. Positioning is accomplishedusing a stinger and landing shoe (See FIG. 4 a) to eccenter the RIT 10within the sub 30. It will be appreciated by those skilled in the artthat other eccentering systems may be used to implement the invention.

As shown in FIG. 22 a, the inductive couplers 122 have “U” shaped coresmade of ferrite. The ferrite core and windings are potted infiberglass-epoxy, over molded with rubber 131, and mounted within acoupler package 130 formed of metal. The coupler package 130 may beformed of stainless steel or a non-magnetic metal. Standard O-ring seals132 placed around the inductive coupler package 130 provide a hydraulicseal. The inductive couplers 122 in the RIT 10 may also be potted infiberglass-epoxy and over molded with rubber 131. A thin cylindricalshield made of PEEK or PEK may also be placed on the OD of the sub 38 toprotect and secure the coupler package 130 (not shown).

In operation, there will be a gap between the inductive couplers 122 inthe RIT 10 and the sub 30, so the coupling will not be 100% efficient.To improve the coupling efficiency, and to lessen the effects ofmis-alignment of the pole faces, it is desirable for the pole faces tohave as large a surface area as possible.

FIG. 22 b shows a 3.75-inch long by 1-inch wide slot 38 in the sub 30.The pole face for this inductive coupler 122 is 1.1-inches long by0.75-inch wide, giving an overlap area of 0.825 square inches. Thisconfiguration maintains a high coupling efficiency and reduces theeffects due to the following: movement of the RIT 10 during drilling ortripping, variations in the gap between the inductive couplers 122, andvariations in the angle of the RIT 10 with respect to the sub 30.Another advantage of a long slot 38 design is that it provides space forthe pressure feed-throughs 124 in the inductive coupler package 130.

Antenna tuning elements (capacitors) may also be placed in this package130 if needed. It will be appreciated by those skilled in the art thatother aperture configurations may be formed in the walls of the sub 30to achieve the desired inductive coupling, such as the circular holesshown in FIG. 20.

Since the pressure inside the sub 30 will be 1-2 Kpsi higher thanoutside the sub 30 in most cases, the inductive coupler package 130should be mechanically held in place. Turning to FIG. 23, the antennashield 128 can be used to retain the inductive coupler package 130 inplace. The shield 128 having slots over the antenna 12 as describedabove, but solid elsewhere. The solid portion retains the inductivecoupler package 130 and takes the load from the differential pressuredrop. Tabs may also be placed on the outside of the inductive couplerpackage 130 to keep it from moving inward (not shown). The shield 128may also be threaded on its ID, with the threads engaging matching“dogs” on the sub 30 (not shown).

FIG. 24 shows a simple circuit model for an embodiment of the inductivecoupler and transmitter antenna of the invention. On the RIT 10 side,the current is I₁, and the voltage is V₁. On the sub 30 side, thecurrent is I₂ and the voltage is V₂. The mutual inductance is M, and theself-inductance of each half is L. This inductive coupler is symmetricwith the same number of turns on each half. With the direction of I₂defined in FIG. 24, the voltage and currents are related byV₁=jωLI₁+jωMI₂ and V₂=jωMI₁+jωLI₂. The antenna impedance is primarilyinductive (L_(A)) with a small resistive part (R_(A)),Z_(A)=R_(A)+jωL_(A). Typically the inductive impedance is about 100Ω,while the resistive impedance is about 10Ω. A tuning capacitor (C) maybe used to cancel the antenna inductance, giving a RIT side impedanceZ₂=R_(A)+jωL_(A)˜j/ωC˜R_(A). The ratio of the current delivered to theantenna to the current driving the inductive coupler isI₂/I₁=−jωM/(jωL+R_(A)+jωL_(A)−j/ωC). The inductive coupler has manyturns and a high permeability core, so L>>L_(A) and ωL>>>R_(A). To goodapproximation, I₂/I₁=˜−M/L (the sign being relative to the direction ofcurrent flow in FIG. 24).

4.8 Implementations of the Invention

As described above, the RIT 10 may be equipped with internal datastorage means such as conventional memory and other forms of the kindwell known in the art or subsequently developed. These storage means maybe used to communicate data and/or instructions between the surface andthe downhole RIT 10. Received signal data may be stored downhole withinthe storage means and subsequently retrieved when the RIT 10 is returnedto the surface. As known in the are, a computer (or other recordingmeans) at the surface keeps track of time versus downhole position ofthe sub so that stored data can be correlated with a downhole location.Alternatively, the signal data and/or instructions may be communicatedin real-time between the surface and the RIT 10 by LWD/MWD telemetry asknown in the art.

FIG. 25 illustrates a flow diagram of a method 300 for transmittingand/or receiving a signal through an earth formation in accord with theinvention. The method comprises drilling a borehole through the earthformation with a drill string, the drill string including a sub havingan elongated body with tubular walls and including at least one stationhaving at least one slot formed therein, each at least one slot fullypenetrating the tubular wall to provide a continuous channel for thepassage of electromagnetic energy 305; engaging a run-in tool within thesub, the run-in tool being adapted with signal transmitting means and/orsignal receiving means 310; locating the run-in tool within the sub suchthat at least one signal transmitting or receiving means is aligned withat least one slotted station on the sub 315; and transmitting orreceiving a signal through the formation, respectively via thetransmitting or receiving means 320.

FIG. 26 illustrates a flow diagram of a method 400 for measuring acharacteristic of an earth formation surrounding a borehole in accordwith the invention. The method comprises adapting a downhole tool withat least one signal transmitting means and at least one signal receivingmeans 405; adapting the downhole tool with end means capable ofaccepting a fishing head or a cable connection 410; and with the fishinghead on the tool, engaging the tool within a drill string to measure theformation characteristic, utilizing the transmitting and receivingmeans, as the drill string traverses the borehole; with the cableconnection on the tool, connecting a cable to the tool and suspendingthe tool within the borehole to measure the formation characteristicutilizing the transmitting and receiving means 420.

The method 400 of FIG. 26 may be implemented with the run-in tools 10and subs 30 of the invention. The run-in tool may be configured with anend segment or cap (not shown) adapted to receive the previouslydescribed fishing head or a cable connection. With the fishing headconnected to the run-in tool, the tool may be used in accord with thedisclose implementations. With the cable connection, the run-in tool maybe used as a memory-mode wireline tool.

It will be understood that the following methods for sealing an openingor slot on the surface of a tubular are based on the disclosed pressurebarriers and slot inserts of the invention.

FIG. 27 illustrates a flow diagram of a method 500 for sealing anopening on the surface of a tubular, wherein the tubular has anelongated body with tubular walls and a central bore. The methodcomprises placing an insert within the opening, the insert being formedin the shape of the opening 505; and applying a bonding material to theinsert and/or opening to bond the insert within the opening 510.

FIG. 28 illustrates a flow diagram of a method 600 for sealing a fullypenetrating opening on the surface of a tubular having an elongated bodywith tubular walls and a central bore. The method comprises placing aninsert within the opening, the insert being formed in the shape of theopening 605, and placing retainer means within the tubular to supportthe insert against the opening 610.

While the methods and apparatus of this invention have been described asspecific embodiments, if will be apparent to those skilled in the artthat variations may be applied to the structures and in the steps or inthe sequence of steps of the methods described herein without departingfrom the concept and scope of the invention. For example, the inventionmay be implemented in a configuration wherein one RIT/sub unit isequipped to measure a combination of formation characteristics,including resistivity, porosity and density. All such similar variationsapparent to those skilled in the art are deemed to be within thisconcept and scope of the invention as defined by the appended claims.

1. A downhole resistivity tool, comprising: a collar having one or moreslots therethrough; and a retrievable sonde movable within a centralbore of the collar, the sonde having one or more antennae, each antennabeing covered by an insulating sleeve and a focusing shield, thefocusing shield having slits therein, the focusing shield configured tofocus electromagnetic signals received by or transmitted by the one ormore antennae into one or more slots of the collar.
 2. The downholeresistivity tool of claim 1, further comprising a pressure barriermounted to the collar.
 3. A resistivity tool for use in a borehole,comprising: a longitudinally elongated tubular structure comprising ahousing located in the borehole and having exterior and interiorsurfaces and one or more stations of slots extending completely throughthe structure between the exterior surface and the interior surface; anda sonde moveably disposed within the interior of the housing, having: atransmitter antenna positioned proximate a certain station of slots; areceiver antenna positioned proximate a certain other station of slots;an electronics module; for each transmitter and receiver antenna, aninsulating sleeve extending about the antenna and a focusing shieldhaving slits therethrough extending about the insulating sleeve, thefocusing shield configured to focus electromagnetic signals transmittedby and received by each respective transmitter and receiver antenna intoone or more stations of slots of the housing.